Chemistry Reference
In-Depth Information
90°
echo
rf
G
diff
(a) The Gradient - Echo Diffusion Weighted Pulse Sequence
90°
180°
echo
rf
G
diff
(b) The Spin - Echo Diffusion Weighted Pulse Sequence
Figure 3.
A simple timing diagram of the gradient-echo and spin-echo pulse sequences, showing
the additional gradient pairs that sensitize them for diffusion.
magnetization,
φ
1
which depends on the spin position [32], namely,
γ
δ
0
φ
1
=
Gx
1
dt
=
γGδx
1
(61)
where the field gradient is applied along the
x
-direction, and the duration of the
gradient lobe is
δ
. The second gradient pulse produces a phase shift
γ
2
δ
δ
φ
2
=
Gx
2
dt
=
γGδx
2
(62)
in the opposite direction. Hence, the net dephasing is given by
φ
=
φ
2
−
φ
1
=
γGδ
(
x
2
−
x
1
)
(63)
x
2
, so that no net de-
phasing occurs. If the spin-carrying liquid molecules undergo translational motion
during the application of the gradients, then there will be net dephasing, which
will affect the transverse magnetization. In other words, static spins are rephased
by the second gradient lobe, while spins diffusing along the direction of the
Spins for whom no change in position occurs have
x
1
=
